It has long been desired to provide reliable, inexpensive security monitoring for shipping containers, especially those used in international shipping. Recently, security concerns have heightened considerably, and the need for preventing weapons of mass destruction, explosives, drugs and other contraband from entering ports or other border crossings has become urgent. Of course, it is also a goal to prevent the theft of goods during shipping.
Prior workers have employed container door seals to detect tampering. Some have employed electronic seals to store information whenever the container doors are opened. Wireless, battery-operated systems have also been employed to communicate the stored data to readers at ports of entry. However, such seal systems fall short of the needs. For example, thieves can defeat seals by removing the container doors from their hinges, and prior wireless systems typically employ high-current consuming two-way communications requiring frequent battery replacement.
Other sensing technologies have also been employed in the past. For example, passive Infrared (“PIR”) sensors can detect warm bodies in the containers if the bodies have a sufficiently different emissive temperature from the background emissive temperature. However, the interior areas of shipping containers are often hot, which condition can cause failure to detect intrusion by a human being. Also, because the containers are subject to external heating, PIRs can falsely indicate the presence of a human being. Moreover, PIRs can easily be blocked by the contents of the container or by dirt, thus shielding the field of view of the PIRs from a human being attempting entry through a hole in a side of the container. In worst case situations, contents or dirt can even block detection of entry through the container doors.
Ultrasonic motion sensors have also been employed within shipping containers. However, they are even more prone to deterioration caused by dirt, moisture, and heat. As with PIRs, dirt and moisture can cause sensitivity deterioration. However, with ultrasonic sensors, heat radiated into the container from the container walls creates heat waves that can reflect ultrasonic energy and thereby cause false alarms.